iio: adc: Add module device table for autoloading

Add the module device id table so that the driver can be automatically
loaded once the platform device is created.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>